Polypropylenes are used in (co)extrusion coating applications primarily for their resistance to permeation by oils and fats and/or for their high gloss surface finish. The use of polypropylenes for high gloss extrusion coatings is well known. The unique high gloss characteristic of polypropylenes is used to enhance the aesthetics of many flexible packages, e.g., wrappers for foods, chewing gum, and clothing. The wrappers' eye-catching appeal is considered a selling point in the retail trade. (See CONVERTING MAGAZINE, February 1992 "Extrusion Resin Review" on page 48).
Polypropylenes have one serious deficiency, however. They contain a heavy loading of stabilizers which are essential in preventing chemical, thermal, and mechanical degradation of the inherently unstable polypropylene molecules. These stabilizers further prevent the polypropylene coating surfaces from readily accepting oxidative treatment, e.g., flame impingement and/or corona discharge. Oxidizing the surface of any inert polyolefin coating is essential to its accepting printing inks. Thus, a package that uses polypropylene as its high gloss outer layer must utilize a second layer beneath the polypropylene if the package is to be printed. That second layer must be a composition, unlike polypropylene, that will readily accept oxidation to permit printing. Polyethylenes are commonly used as this second printed layer because they are chemically stable, and therefore require no stabilizers which could preclude oxidation prior to printing.
Although a polyethylene without stabilizer accepts oxidation and printing, extrusion coating grade polyethylenes do not produce coatings with a high surface gloss comparable to polypropylene. Thus the need for a glossy layer of polypropylene. Film grade polyethylenes have a somewhat better gloss than extrusion coating grade polyethylenes but are still not comparable in gloss to polypropylene, and film grade polyethylenes, with their low swell ratios, defined herein after, are not considered extrusion coatable (see for example U.S. Pat. No. 4,427,833, column 4, lines 1-5).
It would be very desirable to be able to produce a high gloss package that would contain a single "dual purpose" outer layer of a composition that would provide high gloss comparable to polypropylene, but yet accept oxidation to allow printing.
Vinyl cling films are vinyl-backed photographs and/or cartoons, e.g., that can be applied to smooth surfaces such as metal or glass. These vinyl cling films are currently supported on a protective backing that is a high gloss polypropylene coated paperboard. The high gloss polypropylene surface provides an excellent cling surface for these vinyl cling films. However, these cling films dislodge or slide in a parallel motion over these smooth polypropylene surfaces. An oxidized polyethylene surface solves this sliding problem, however, the vinyl cling film does not readily cling to these polyethylene surfaces since their gloss is not comparable to polypropylene. Thus, it would be very desirable to be able to have a single dual purpose outer layer for use with vinyl cling films that would provide both the high gloss surface needed for cling and the oxidized surface needed to prevent sliding or dislodgement.